Active Ripple Rejection of Renewable-Energy PWM Converters

Within the applied research project a novel active ripple current compensation unit for improving the EMC behavior of mains connected PWM converters shall be developed and verified. Such PWM converters - which are frequently used for the coupling of renewable energy sources like solar, wind or fuel cell systems to the mains - in the not to low power area usually are based on IGBT semiconductor devices employing switching frequencies in the region 2...20kHz. In general, the appearing switching frequency ripple current has to be rejected by proper mains side filtering resulting, however, in bulky EMC filters and additional losses. The underlying concept of the project now is to replace the passive ripple filter by an active system which, in the ideal case, completely eliminates (i.e., compensates) the IGBT converter`s ripple. For this, however, a current injector showing a considerable bandwidth (typ. 20...100kHz) will be required to achieve a good rejection behavior. Because this can not be realized by application of standard switch-mode amplifier techniques for the aimed power level, a novel multi-cell amplifier topology based on a series arrangement of low-voltages switching cells is introduced. Due to the low operating voltage of the individual cells modern high-current trench MOSFETs can be used resulting in low on-state losses. Furthermore, the cells advantageously are operated in an interleaved PWM mode which gives a very high total effective switching frequency and guarantees the required dynamic behavior. Besides a basic analog control, also of a fully-digital control (including the required ripple signal filter) is planned to be implemented. Furthermore, a specific type of a Rogowski-sensor shall be analyzed for sensing the IGBT converter`s ripple current.

Within the applied research project a novel active ripple current compensation unit for improving the EMC behaviour of mains connected PWM converters was developed and verified. Such PWM converters which are frequently used for coupling of renewable energy sources like solar, wind or fuel cell systems to the mains in the not too low power area usually are based on modern semiconductor devices (IGBT) employing switching frequencies in the region 2..20kHz. In general the appearing switching frequency ripple current must be rejected by proper mains side filtering resulting, however, in bulky EMC filters and remarkable additional losses. The underlying concept of the project was to replace the passive ripple filter by an active system which, in ideal case, completely eliminates (i.e. compensates) the power-converter ripple current. To fulfil this goal a current injector with considerable bandwidth (typ. 20..100kHz) is required to achieve a good rejection behaviour. Because this cannot be realized by standard switching mode amplifier techniques for the aimed power level, a multi-cell amplifier topology based on a cascaded arrangement of a low-frequency and a high-frequency compensator was chosen. The goal of the first compensator cell was to eliminate the switching ripple of the power inverter, while the high-frequency stage was used to eliminate the much smaller switching ripple of the first stage. The whole concept was easy to control due to clearly defined goal-functions for each stage. Moreover it gives a rather simple approach for fully digital solutions.